The long term goals of this program project on Hearing Development are to understand the normal ontogeny of human hearing and the biological mechanisms underlying the occurrence of hearing disorders during development. The first steps toward achieving these goals are a more thorough understanding of the time course of hearing development in normal human infants, an understanding of structure-function relationships underlying auditory system ontogeny and animal studies investigating the cellular mechanisms responsible for normal and abnormal development. Toward these ends we have proposed eight interrelated research projects, all of which focus on development of hearing and the vertebrate auditory system. Six of the projects investigate hearing and perceptual development in human infants, while two are concerned with the cellular mechanisms regulating development of central nervous system auditory neurons. A variety of techniques will be used to derive converging information on auditory development in human infants. For example behavioral studies, physiological experiments and acoustical measurements of external and middle ear will all address developmental changes in sensitivity and frequency selectivity as a function of sound frequency. Similarly, changes in temporal processing will be assessed by behavioral and physiological methods. Each of these studies provides important information for understanding the development of language processing and production, which will be investigated by Kuhl and Stoel-Gammon, who will also investigate the uniqueness of language processing using non-human primates. In addition the application of new physiological techniques to hearing disabled children will be assessed. At a more cellular level we will continue investigating interactions between peripheral dysfunction and central nervous system development using morphological, immunohistochemical and biochemical methods. The entire program is built on the philosophy that a number of well equipped laboratories headed by senior investigators in close communication can use complimentary approaches and share resources to significantly advance our understanding of hearing development.